https://doi.org/10.15255/KUI.2003.001
Published: Kem. Ind. 53 (3) (2004) 117–123
Paper reference number: KUI-01/2003
Paper type: Review
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Electrospray – Soft Ionization in Mass Spectrometry

N. Galić

Abstract

Aspects of the major processes by which ions in solution are converted to ions in the gas phase are given on the basis of information available in the articles on electrospray mass spectrometry (ESMS).1-14 The solution containing the analyte is introduced with a weak flux (normally 1 – 10 μl min-1) in a capillary on which a high electric field is applied under atmospheric pressure (Figure 1). The field causes a separation of positive and negative charges in the solution. In the positive ion mode (when the capillary is the positive terminal), positive ions tend to move towards the counter-electrode and accumulate at the surface of the liquid at the capillary tip. At a critical field the meniscus at the tip deforms into a liquid cone, called a Taylor cone, which continuously produces droplets (Figure 2). The droplets’ surfaces are enriched with positive ions for which there are no negative counterions in the droplet. Reversing the polarity of the power supply can generate negatively charged droplets instead. As electrospray produces a continuous current, redox processes must occur at the capillary and at the counter electrode to avoid charge accumulation. In other words, electrospray device can be viewed as a special type of electrolytic cell in which part of the ion transport does not occur through uninterrupted solution, but as charged droplets and later as ions in the gas phase. Evaporation of solvent from the initially formed droplets, as they traverse a potential and a pressure gradient towards the analyzer of the mass spectrometer, leads to a reduction in size. The radius of the droplet decreases at constant charge until being close to Rayleigh limit, when the Coulombic repulsion between the charges overcomes the cohesive forces. This leads to the Coulomb fission of the droplet (Figure 3). Small offspring droplets have radii which are roughly one-tenth of the parent droplet radius. They carry away approximately 2 % of the parent mass and 15 % of the parent charge. As evaporation carries on, the daughter droplets undergo fission themselves. Two different mechanisms have been proposed to account for the formation of desolvated ions in the gas phase from the small charged droplets. The charged residue model, proposed by Dole,15 assumes that the series of droplet fission events leads to the extremely small droplets containing only one ion. Molecules having masses above at least 3300 Da are produced via this mechanism.9,18 According to the ion evaporation model, proposed by Iribarne and Thomson,16,17 at an intermediate stage in the droplets’ lifetime (prior to the Rayleigh limit) the electric field on the surface of the charged droplets is sufficiently high so that solvated ions may be emitted directly into the gas phase. It is now generally admitted that small ions are produced predominantly by this mechanism.6,19,20 Of major concern to the mass spectrometrist is whether the relative intensities in the MS spectra reflect the relative abundances of the analytes present in solution. The intensity of the signal corresponding to the analyte depends on its concentration and on its response. The effect of the electrospray mechanism on the response of the analyte is decribed by Tang and Kebarle21,22 and G. Enke.23 Only the key features are described in this paper. The ions observed in the gas phase may be different from those present in the solution. Transfer of the ions from solution to gas phase without chemical change can occur only for very stable ions such as singly charged alkali ions. Very important changes can occur when the ions involved are protonated bases or deprotonated acids. The term "wrong-way-round ionization" has been used to describe the observation of protonated or deprotonated ions electrosprayed from solutions where such ions are not expected to exist in appreciable concentrations.24-27

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Keywords

mass spectrometry, electrospray, mechanisms, review